Pulsed laser or LED diode drivers (or current drivers) are used to generate pulses of current into a single or typically a series array of laser diodes. The techniques described herein may be applicable to any Laser Diode, LED or similar current-driven load (or device). The light output of an LED or laser diode may be used for various purposes, such as automobile LIDAR (Light Detection and Ranging), rangefinding, or as a light source requiring a short pulse with a high peak power.
As is known, inductors generally oppose changes in current. When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor, described by Faraday's law of induction. According to Lenz's law, the direction of induced electromotive force (emf) opposes the change in current that created it.
In some applications, such as described herein, it may be desirable to have a high-speed efficient laser diode driver capable of driving 100 amps or more in a very short time, for example 10 ns. A fundamental problem with this is the inductance of the laser diode, or diode array where multiple laser diodes are connected in series. This inductance may be up to 10 nH or higher even using low inductance practices.
Typically, laser diodes may be damaged by a few volts in the reverse direction and, in the current state of the art, they may conventionally be protected by a fast diode across the laser diode terminals to conduct in the reverse voltage direction. Such a diode may be referred to as a “shunt protective” (or simply “shunt”) diode. For short, high-current pulses, this has the undesirable effect of providing a long tail at turn-off as the parasitic inductance dissipates its stored energy through the shunt protective diode at a low voltage, and therefore over a long time interval. It would therefore be desirable to remove the stored energy at a high voltage so that the inductor discharge current can only flow for a very short time using the general relationship:Time=Inductance×Current(di)/Voltage.
As is evident from the above, a high voltage therefore demands (or results in) a short time interval.
Some Patents and Publications
U.S. Pat. No. 6,697,402 (Analog Modules, Inc.) discloses high-power pulsed laser diode driver, and shows a method of driving the laser inductance quickly using a pre-charged inductor that generates an instantaneous high voltage. More specifically, fast rise time to high currents in a load such as a laser diode array is achieved by connecting an inductor between a power supply and an end of the diode array. A switching element, is connected between the other end of the diode array and ground. A shunt switch is connected across the diode array. When the shunt switch is opened, energy stored in the inductor is suddenly delivered to the diode array. A diode may be connected between the other end of the diode array and the input of the driver. A current monitor may be connected in series with the diode array. An overall system comprises the diode array driver(s) and at least a portion of the power supply—namely, an energy storage capacitor. A value for the energy storage capacitor in the power supply may be selected to produce a maximally flat-top pulse shape. A source voltage provided by the power supply may be greater than, substantially equal to, or less than the voltage required by the diode array. In use, closing the switching element and closing the shunt switch produces an initial current buildup in the inductor, and opening the shunt switch directs the current built up in the inductor into the diode array. Current flow through the diode array is terminated by subsequently closing the shunt switch. With the shunt switch closed, the switching element may be opened, which will cause the current in the inductor to recirculate within a loop comprising the closed shunt switch, the inductor and the diode connected across the series-connected diode array and the coil. Periodically closing the switching element will refresh the recirculating current. Refreshing the current in the inductor for a burst, or very short lead time, may be done by turning on (closing) the switching element for a short time with the shunt switch closed, until the current sensed rises to the desired value.
As further disclosed in U.S. Pat. No. 6,697,402, in use, a first switch is turned on to build up current in the inductor just prior to the laser pulse. During this period, the load (diode array) is shorted out by a second (shunt) switch in series with the first switch. When the desired peak current is reached, the second switch is turned off (opened). Because current flow in an inductor can not change abruptly, the current continues to flow into the load, generating a high voltage, as required, to overcome the reactance of the load and leads. To turn off the pulse, the second switch is turned on, shorting out the load and discharging the lead/load inductance as the falling edge of the pulse. Simultaneously, the first switch is turned off and the energy stored in the series inductor is recycled back into the storage capacitor through a diode.
US 20170085057 (Analog Devices, Inc.) discloses pulsed laser diode driver and a similar (to the aforementioned U.S. Pat. No. 6,697,402) pre-charge solution. Optical systems can emit train(s) of light pulses onto objects to derive a distance between the light source and the object. Achieving meter or centimeter resolution may require very short light pulses. It is not trivial to design a circuit that can generate narrow current pulses for driving a diode that emits the light pulses. A driver circuit has a pre-charge path comprising one or more inductive elements and a fire path comprising the diode. Switches in the driver circuit are controlled with predefined states during different intervals to pre-charge current in the one or more inductive elements prior to flowing current through the fire path to pulse the diode.
U.S. Pat. No. 7,262,584 (Analog Modules, Inc.) discloses efficient fast pulsed laser or light-emitting diode driver, and shows a charged capacitor switched into a laser diode. A capacitor is connected to the output of a multiphase power converter, and a current-driven device (e.g., LED or laser diode) is also connected to the power converter output. A solid state switch (FET or IGBT) is connected in series with the current-driven device. Means are provided for sensing current through the current-driven device. An error amplifier compares sensed current through the current-driven device with a current level demand signal and controls the output of the power converter. Means are provided for turning the switch on and off and may be (i) a fast comparator receiving a voltage reference signal at one input and the current level demand signal at another input, and outputting the switch on/off signal to the switch or (ii) an externally-generated logic signal provided directly to the switch.
U.S. Pat. No. 8,184,670 (Analog Modules, Inc.) discloses smart linear pulsed laser diode driver, and method, and shows that the capacitor voltage may be adjusted to control the efficiency of the laser diode driver. In a pulsed laser diode driver an energy storage capacitor is continuously being charged to a supply voltage Vr. When a pulse is initiated, energy stored in the capacitor is delivered to the laser diode load. The capacitor voltage Vd at the end of a pulse is used to control Vr to ensure that Vd is maintained above a minimum voltage Vm required to ensure operation of a current control device (such as an FET) just above saturation. Test pulses (such as with attenuated currents or reduced pulse width) may be fired to determine an initial optimum value for Vr. After a test pulse, a slightly high estimate for Vr may be used and may be iterated (incremented) down to an optimum value Vm during a firing burst. A digital processor may be used to calculate and store data to optimize the performance. Various embodiments are disclosed. This illustrates the concept of adjusting the power supply voltage to assist in regulation, but strictly does not apply to a current-controlled device by definition.
U.S. Pat. No. 7,545,839 (Optiswitch Technology Corporation) discloses apparatus and method for driving a pulsed laser diode, and specifies a high starting voltage to achieve a fast rise time followed by a lower voltage slow discharge to maintain the current efficiently. To achieve both a fast risetime and a desired flat top current pulse, or to be able to independently specify a risetime and pulse width (energy), a supplemental or “fast” voltage discharge stage (or multiple supplemental or “fast” voltage discharge stages) having a faster and shorter voltage discharge characteristic and a higher starting voltage relative to the main or “slow” voltage discharge stage is used in parallel with the slow voltage discharge stage. The energy storage element of the slow voltage discharge stage has sufficient energy storage at an appropriate voltage level for maintaining the desired flat top current throughout the pulse duration, while the energy storage element of the fast voltage discharge stage has less energy storage capability but a higher starting voltage for achieving the desired fast current pulse risetime. Preferably, a single closing switch is used to couple all energy storage elements to the laser diode to pulse it, although respective separate switches may be used to couple the energy storage elements of the various voltage discharge stages to the laser diode.